1. Field of the Invention
This invention relates generally to processing of amplitude modulated (AM) signals and more particularly to the processing of AM signals prior to application of the signals to a broadcast unit in order to improve the quality of the stereophonic sound reproduced at the receiving apparatus.
2. Description of Related Art
In the broadcast of amplitude modulated (AM) stereo signals, government regulations require that the broadcast transmission be comprised of a carrier amplitude modulation which is encoded with the sum of the left channel stereo signal and the right channel stereo signal, the carrier further having phase modulation which is encoded with the difference between the left channel stereo signal and the right channel stereo signals. To the extent possible, it is desirable that the left channel stereo signal have 100% amplitude when that signal alone is processed by the receiving apparatus, that the right channel stereo signal have 100% amplitude when the right detection signal is processed by the receiving apparatus, and that both signals have 100% amplitude when processed by the receiving apparatus (monaural) operation.
In a conventional left and right limiting signal pattern, shown in FIG. 1a, the sum of the left channel stereo signal and right channel stereo signal (L+R) signal represents the main monaural component at the broadcast transmitter. The difference between the left channel stereo signal and the right channel stereo signal (L-R) represents the stereophonic information to be transmitted.
Referring once again to FIG. 1a, when the input signal from a stereo unit shifts to full right channel signal or to full left channel signal, the L+R modulation component is forced to drop 50% as shown by the dotted line, and thus the monaural component drops by 50% (6 db). Although the stereo reproduction is not affected, the monaural reproduction is deemed to be unacceptable.
To reduce the effect of full right channel or full left channel communication, full matrix limiting has been utlilzed and is shown in FIG. 1b. With this system the output signal levels of the L+R and L-R are adjusted for equal modulation, which is the point of maximum separation. As shown, the amplitude limit levels are perpendicular to the L+R and L-R signals and intersect at the left signal and right signal axis. When the stereo inputs temporarily shift to full right channel or full left channel modulation, the limit levels permit the L+R signal component to remain at 100% modulation, which maintains full monaural reception compatibility. The shaded areas shown in FIG. 1b illustrate the increased monaural support modudlation as compared to earlier systems. However, further analysis demonstrates that stereo reception will have a 50% (6 db) increase in signal channel receptions. While this change has been shown to be more acceptable to listeners than the equivalent change in monaural reception, a need was felt for a system that provided a more accurate transmission of the stereo signals.